1. Field of the Invention
The present invention relates to a liquid ejection head substrate and a liquid ejection head. Specifically, the present invention relates to an ink jet head substrate and an ink jet head including a foaming heater that allows ink to be foamed and a sub-heater that preheats a substrate.
2. Description of the Related Art
In typical thermal liquid ejection heads (hereinafter also referred to as heads), a liquid ejection heater (hereinafter also referred to as a heater) that generates energy for ejecting liquid and a conductive layer that supplies electricity to the heater are disposed on a substrate. A channel member that defines a channel communicating with an ejection port configured to eject liquid is disposed on the substrate.
In recent years, some thoughts have been put into a liquid ejection head substrate (hereinafter also referred to as a head substrate) to stabilize liquid ejection. For example, there is a technology in which a substrate-heating heater (hereinafter also referred to as a sub-heater) that preheats a substrate is independently disposed in addition to an ejection heater.
Japanese Patent Laid-Open No. 3-005151 discloses that the degradation of ejection characteristics caused at a low temperature is prevented by forming a heater and a sub-heater both composed of the same material in the same layer and by heating a head substrate using the sub-heater.
The reliability of a sub-heater will be described. When a conductive layer (hereinafter also referred to as a wiring sub-heater) is used as a sub-heater by supplying a high current to aluminum (Al) commonly used as a conductive layer, attention needs to be paid to electromigration durability.
Electromigration (hereinafter also referred to as E. M.) is a phenomenon in which, by supplying an electric current to a conductive layer, aluminum (Al) atoms constituting the conductive layer move in a direction in which electrons flow. As a result, voids (holes), hillocks (bumps), and whiskers (whisker-shaped growth) are produced. It is widely known that mean time to failure due to E. M. is expressed using Black's empirical formula. According to Black's empirical formula, the mean time to failure due to E. M. is normally inversely proportional to the nth power of current density (normally n is 2, which depends on a temperature gradient, accelerating conditions, or the like). In other words, when a wiring sub-heater is used, current density needs to be reduced to a certain value or less to achieve a sufficiently long life against E. M.    Reference: Black's empirical formulaMTTF=A×J−n×eEa/kT     MTTF: mean time to failure (hour)    A: a constant determined in accordance with a structure and a material of a conductive layer    J: current density (A/cm2)    n: a constant representing current density dependence    Ea: activation energy (eV) (normally 0.4 to 0.7 eV, which depends on orientation, a particle size, a protective layer, or the like)    k: Boltzmann's constant 8.616×10−5 eV/K    T: absolute temperature of a conductive layer (K)
To use a wiring line composed of a conductive layer as a sub-heater, power consumption that exceeds a certain value is needed. To secure the required power consumption and to achieve a long life against E. M., current density needs to be reduced while a constant resistance is maintained. Consequently, the wiring line needs to be lengthened and the sectional area needs to be increased. For example, when the length of the wiring line is doubled and the sectional area is doubled, the power consumption does not change because the resistance of the wiring line constituting the wiring sub-heater does not change. On the other hand, since the current density can be reduced by one half, the mean time to failure can be lengthened to a value that is about four times the original value according to Black's empirical formula.
As described above, in the wiring sub-heater, the wiring line needs to have an appropriate length and sectional area to achieve a long life against E. M. Furthermore, to preheat a substrate in a uniform temperature distribution, the wiring line constituting the sub-heater should be uniformly arranged on a head substrate when viewed in plan.
To secure the wiring line having an appropriate length and dispose the wiring sub-heater on a head substrate in a substantially uniform manner, it is effective to constitute the wiring sub-heater using a plurality of conductive layers.
In view of the foregoing, the inventors of the present invention performed E. M. durability investigation, which posed a problem in that the E. M. durability at a connecting portion (111) of an insulating layer that is a contact portion of conductive layers is poorer than that in a region of a conductive portion (112).
According to Black's empirical formula, an area of the connecting portion can be increased to improve the E. M. durability at the connecting portion as described above. However, an unnecessarily large connecting portion increases a substrate size.